Synchronous rotating electrical machine with permanent magnets and flux concentration

ABSTRACT

A synchronous rotating electrical machine is disclosed, of the type including a stator ( 10 ) and a rotor ( 11 ). The rotor is of the flux concentration type and includes a plurality of alternate North and South poles formed from permanent magnets (PM). The magnets are housed in slots (E 1 ) arranged in the magnetic body of the rotor. Each pole comprises a second slot (E 3 ) roughly radial and arranged in the magnetic body of the pole, between two consecutive magnets delimiting the pole. In accordance with the invention, each pole includes a portion forming a bridge (BR) between the magnetic bodies of the rotor, on either side of the second slot, this part forming a bridge including a magnetic body height not greater than approximately one times the dimensional value of the width of the permanent magnets.

The present invention relates to a synchronous rotating electricalmachine with permanent magnets and flux concentration. Moreparticularly, the invention relates to a rotating electrical machine ofthis type for applications such as generators or electric tractionengines in electric and hybrid automotive vehicles.

Thanks to their improved performance in terms of output andpower-to-weight and power-to-volume ratios, synchronous engines withpermanent magnets are widely used today in the field of traction inautomotive vehicles. Moreover, the availability of rare-earth permanentmagnets on a large scale and under acceptable economic conditions makesthe choice of such electric engines viable for new generations ofautomotive vehicles.

Such electric engines can be produced in a wide range of powers andspeeds and will find applications in all-electric vehicles and in lowCO₂ vehicles of the types known as “mild-hybrid” and “full-hybrid”.

“Mild-hybrid” applications generally concern electric engines of theorder of 8 to 10 KW, for example, an electric engine mounted at thefront of a heat engine and coupled to it by a drive belt. It is possiblewith such an electric engine to reduce the thermal cubic capacity(engine downsizing) by providing torque electrical assistance whichsupplies auxiliary power especially when accelerating. Moreover,low-speed traction, for example in an urban environment, can also beprovided by this same electric engine. Applications of the “full-hybrid”type generally concern engines of 30 to 50 KW for architectures of theseries and/or parallel type with a more successful level of integrationof the electric engine or engines in the vehicle's traction chain.

Among the known different synchronous engines with permanent magnetsthose of the flux concentration type are of particular interest owing totheir excellent performance. In these flux concentration engines, themagnets are buried in the magnetic body of the rotor and arrangedaccording to a roughly radial configuration.

In an automotive vehicle, an electric engine used in traction on all ofthe vehicle's circulation tasks is subject to variable conditions ofspeed and charge. A strategy of maximum torque control complemented by adefluxing strategy (also called “demagnetisation strategy”) to achievethe high speed zone seems to be a good solution for controlling theelectric engine.

To achieve the above objective, it is desirable to provide synchronousrotating electrical machines with permanent magnets and fluxconcentration which are optimised with torque and mechanical inertia.Maximum mechanical torque must be provided by the machine and theminimum inertia of the rotor especially to facilitate increases inspeed.

According to a first aspect, the present invention relates to a rotatingelectrical machine comprising a stator equipped with stator windings anda rotor, the rotor comprising a plurality of alternate North poles andSouth poles which are formed from permanent magnets arranged accordingto a roughly radial configuration known as with flux concentration,first slots arranged in the magnetic body of the rotor and in which arehoused respectively the permanent magnets; the rotor comprising, in eachof its poles, a second slot arranged roughly radially in the magneticbody of the pole concerned, between two consecutive permanent magnetsdelimiting the pole, the second slot participant in a reduction in themechanical inertia of the rotor; the rotor comprising, in each of itspoles, a portion forming a bridge between the magnetic bodies of therotor on either side of the second slot, and the permanent magnets beingroughly rectangular in shape with a height in the radial direction ofthe rotor, a length in the axial direction of the rotor and a widthhaving predetermined dimensional values.

In accordance with the invention, the part forming a bridge comprises amagnetic body height not greater than approximately one times thedimensional value of the width of the permanent magnets.

According to a particular characteristic of the invention, the partforming a bridge comprises a magnetic body height greater thanapproximately one times the unit metal sheet thickness of the packet ofmetal sheets forming the magnetic body of the rotor.

According to a further particular characteristic of the invention, thepart forming a bridge comprises a magnetic body length less thanapproximately one and a half times the dimensional value of the width ofthe permanent magnets. Preferably, the magnetic body length of the partforming a bridge will be greater than approximately one and a half timesthe unit metal sheet thickness of the packet of metal sheets forming themagnetic body of the rotor.

According to a further form of embodiment of the invention, the partforming a bridge is set back in relation to the circumferential surfaceof the rotor.

According to a further particular characteristic of the invention, thesecond slot comprises a top part which participates in a reduction inthe mechanical inertia of the rotor and a bottom part which participatesin controlling a defluxing magnetic flux through the central part of therotor.

According to a further particular characteristic of the invention, thetop part of the second slot has a roughly trapezoidal shape with asmaller side corresponding roughly to the part forming the bridge.

According to a further particular characteristic of the invention, thesecond slot is roughly off-centre, between the two consecutive permanentmagnets delimiting the pole.

According to a further particular characteristic of the invention, therotor comprises, on either side of a said second slot, at least onethird slot contributing to reducing the inertia of the rotor andoriented roughly within the alignment of magnetic field lines.

According to a further aspect, the invention refers to a process fordimensioning a rotor of a rotating electrical machine as briefly definedbelow. In accordance with the invention, the part forming a bridge isdefined dimensionally so as to obtain an optimum compromise between amaximum mechanical torque provided by the machine and a minimummechanical inertia of the rotor.

Other characteristics and advantages of the invention will becomeapparent on reading the following description of a number of particularforms of embodiment, with reference to the figures below, in which:

FIG. 1 shows, in a simplified fashion, the structure of a particularform of embodiment of a defluxable rotating electrical machine withpermanent magnets and flux concentration according to the invention;

FIG. 2 shows the different slots embodied in the magnetic body of therotor of the machine of FIG. 1;

FIG. 3 shows a configuration of mounting of the permanent magnets,incorporating a strip, at the top extremity of the magnets;

FIG. 4 shows a network of reluctances of a defluxing magnetic circuitfor a permanent magnet of the rotor of the machine of FIG. 1;

FIG. 5 is a magnetisation curve of the magnets showing the working pointof these latter at the intersection of their straight line of charge;

FIGS. 6 a and 6 b show respectively first and second embodiments of aportion forming a bridge between the magnetic bodies of the rotor, oneither side of a slot E3;

FIGS. 7 and 8 are respectively torque and inertia curves as a functionof a height H of the portion forming a bridge of FIGS. 6 a and 6 b; and

FIGS. 9 and 10 are respectively torque curves of the machine and inertiacurves of the rotor as a function of a length L of the portion forming abridge of FIGS. 6 a and 6 b.

FIG. 1 shows the structure of a particular form of embodiment 1 of adefluxable rotating electrical machine according to the invention. Themachine 1 is of the type with buried magnets and flux concentration andcomprises a stator 10 and a rotor 11.

A concrete embodiment of such a machine according to the invention isfor example a traction engine of 8 to 10 KW for applications inautomotive vehicles of the type known as “mild-hybrid”. A particularform of embodiment of this engine comprises a stator 10 having 60recesses 101 and a rotor 11 comprising 10 alternate North and Southpoles. The rotor 11 has a diameter of the order of 100 mm and an axiallength of the order of 50 mm. The rotor 11 comprises 10 permanentmagnets PM having a roughly rectangular shape and with the dimensions:length (L_(zs))×height (h_(a))×width (l_(a))=50 mm×25 mm×5 mm.

The stator 10 and the rotor 11 are traditionally equipped with packetsof metal sheets forming magnetic bodies.

The recesses 101 of the stator 10 are provided to receive statorwindings (not shown) and form between them a plurality of stator teeth.Depending on the forms of embodiment, the recesses 101 will be providedto house concentrated windings, wound on large teeth, or distributedwindings.

The rotor 11 has the general shape of a multi-lobed cylinder, each ofthe lobes corresponding to a magnetic pole of the rotor.

The magnets PM are arranged radially so as to obtain a rotor structureof the flux concentration type. In certain forms of embodiment, themagnets PM may be slightly unbalanced in relation to the radius of therotor 11. The magnets PM are preferably rare-earth permanent magnetssuch as magnets of the Neodymium Iron Boron (NeFeB), Samarium-Iron(SmFe) or Samarium-Cobalt (SmCo) type or magnets obtained from sinteredor bonded ferrites.

The rotor 10 comprises a central bore emerging at its two facialextremities and designed to receive its drive shaft A. It will be notedthat in the present invention the shaft A can be made of a magnetic ornon-magnetic material according to the application envisaged.

The rotor 10 also comprises slots E1, E2 and E3 which are repeated foreach pole and extend axially over roughly the entire length of therotor.

Closing metal sheets, without the slots E1, E2 and E3, may be providedat the facial extremities of the rotor 11 in order to contribute to theassembly of the rotor 11. Soldering points (not shown) at the edge ofthe packet of metal sheets and through tie-rods (not shown), parallel tothe central axis, are also provided for the assembly of the rotor 11.The through tie-rods are made of a magnetic or non-magnetic material,depending on the applications. Advantageously, the passage of thetie-rods through the packet of metal sheets of the rotor 11 can be madethrough the slots E3.

In this particular form of embodiment, the slots E1, E2 and E3 are each10 in number, this number corresponding to the number N=10 of magneticpoles of the rotor 11.

The slots E1, E2 and E3 are now described in detail with reference alsoto FIGS. 2 and 3.

The slots E1 form quasi-rectangular housings for the permanent magnetsPM. The slots E1 are not occupied entirely by the magnets PM andcomprise parts left vacant which fulfil functions of reluctance andmagnetic barrier for controlling the passage of the magnetic flux in themagnetic body of the rotor 11 and the magnets PM.

The slot E1 emerges on the external circumference of the rotor 11 bymeans of a recess E10. The recess E10 is extended axially along thelength of the rotor 11 and thus delimits the pole in relation to theneighbouring pole. First and second tips B1, B2 are thus formed inadjacent poles, which are opposite each other and designed to retain thepermanent magnet PM in its housing, by opposing the effect of thecentrifugal force on the magnet PM.

A reluctant space E11 is also provided between the top edge of themagnet PM and the lower face of the tips B1, B2. This reluctant spaceE11 is a space of the slot E1 left vacant by the magnet PM.

FIG. 3 shows details of a mounting of the permanent magnet PM in thetips B1 and B2. This mounting in FIG. 3 corresponds to a particular formof embodiment in which a strip LM is provided.

The strip LM is interposed between the upper face of the magnet PM andthe lower faces of the tips B1, B2. The strip LM is for example made ofcharged plastic, in a resin of the glass fibre charged epoxy resin type,a composite material or a deformable non-magnetic metallic material. Thestrip has the function of spreading the mechanical efforts beingexercised on the top of the magnet PM and the tips B1, B2 and ofabsorbing by deformation any displacement of the magnet PM. The testscarried out by the inventive entity have shown that the centrifugalforces being applied to the magnet PM can be significant. If the machineis subjected to a very high speed of rotation, the magnet PM tends tomove away from the axis of rotation of the rotor 11, under the effect ofthe centrifugal force, and deformation of the tips B1, B2 towards theexterior of the rotor 11 could occur. The strip LM contributes to abetter spread of the mechanical stresses between the tips B1, B2 and byabsorbing by deformation any displacement of the magnet PM thus reducesthe risk of a breakage of the magnet PM and/or of deformation of thetips B1, B2. The tests carried out have shown that a strip thickness ofat least 0.1 mm is desirable for it to correctly fulfil its function.

In accordance with the invention, there is also provided an angle ofinclination a of the lower face of the tip B1, B2, in relation to theupper face of the magnet PM. This inclination can for example beobtained by means of a chamfer which according to the application willhave an angle of between α=0.1° and α=15°.

Preferably, as shown in FIG. 3, there is associated with this chamfer ofangle α a mechanical reinforcement obtained by means of a rounding ofradius R. This rounding is made between the lower chamfered face of thetip B1, B2 and the related face, roughly radial, forming an interiorwall of the magnet housing. The radius R will preferably have a value ofbetween 0.2 and 0.9 times the depth L_(B) of the tip B1, B2. Forexample, the radius R will be between approximately R=0.5 mm andapproximately R=1.5 mm. As can be seen in FIG. 3, chamfers orequivalents (roundings) CF are provided in this form of embodiment onthe upper part of the magnet PM so as to avoid contact on the edge ofthe magnet PM with the rounding of radius R. Reference will be madesolely to the chamfers CF in the rest of the description and in theattached claims, in the knowledge that the term “chamfer” also coversthe equivalents such as roundings.

The tests carried out by the inventive entity have shown that for theclass of machines to which the invention applies, an angle a and aradius R in the ranges indicated above make it possible to obtainsatisfactory results in terms of optimum resistance to the centrifugalforces being exercised in the range of speeds from 0 to 20000 rpm.

The strip LM allows a distribution of the mechanical efforts on the twotips B1 and B2 and can in certain cases make it unnecessary to chamferthe edges of the magnets PM when its thickness is sufficient.

It will be noted that the magnetic reluctances introduced by the recessE10 and the reluctant space E11, as well as participating in the generalpolarisation of the magnet PM, described later, oppose localdemagnetisation of the magnet PM at the edges.

As can be seen more particularly in FIG. 2, the slot E1 comprises in itsbottom part, near to the shaft of the rotor 11, reluctant spaces E12which are, in this form of embodiment, spaces filled with air, leftvacant by the magnet PM and which on introducing a magnetic reluctanceprevent local demagnetisations of the magnet PM.

The slots E2 essentially have a function of reluctance and magneticbarrier for controlling a defluxing magnetic flux through the centralpart of the rotor, in other words, in the magnetic body between thebottom of the magnets PM and the shaft A of the rotor. It will be notedthat these slots E2 are filled with air in this particular form ofembodiment. In certain applications, they can be filled with magnetic ornon-magnetic materials with a low relative permeability.

The slots E3 fulfil a number of functions. Generally speaking, theirfunction is essentially to contribute, like the slots E2, to the controlof the defluxing magnetic flux, through the central part of the rotor,and to reduce the inertia of the rotor 11. In this form of embodiment,like the recesses E10, the reluctant spaces E11, E12 and the slots E2,these slots E3 are filled with air. In certain applications, they canalso be filled with non-magnetic, or magnetic, materials but with lowdensity.

Although the slots E3 are represented here as being arranged radially inthe rotor 11, centred between the consecutive magnets PM and symmetricalin shape, it will be noted that in other forms of embodiment of theinvention, the slots E3 can, especially in their top part, be neithercentred nor symmetrical in shape.

As shown especially in FIG. 2, other slots, labelled E4, are arranged ineach pole, on either side of the slot E3. The slots E4 contribute toreducing the inertia of the rotor 11 and are situated within thealignment of the field lines, between them, so as to oppose to the leastextent possible the passage of the magnetic flux of the magnets PM. Inthis form of embodiment, the slots E4 are two in number on either sideof the slot E3. Generally speaking, the number of the slots E4 can varyaccording to the application and the room available. For example, theirnumber can vary from 1 to 8, although 2 to 3 for the present inventionis a good compromise.

Furthermore, it will be noted that the practical rules for cutting metalsheets impose the use of a width of material depending on the case ofbetween 1 and 2 times the thickness of the metal sheet. In other words,this means, for example, that between two neighbouring slots of therotor or between a slot and an external circumference of the rotor,there must be at least a width of material of between 1 and 2 times thethickness of the metal sheet. Thus, for example, for a metal sheet 0.35mm thick, the minimum width of material to be conserved will be between0.35 mm and 0.7 mm.

In this form of embodiment, the slot E3 comprises an upper trapezium E30and a bottom part E31. Generally speaking, the upper trapezium E30 makesit possible to reduce the inertia of the rotor 11. However it has aneffect on the magnetic reaction of the armature and can also bedimensioned so as to participate in the control thereof. The bottom partE31 is the part of the slot E3 which participates in the control of thedefluxing magnetic flux through the central part of the rotor 11. Thepart E31, jointly with the reluctant spaces E12 and the slot E2, makesit possible to control the passage of the magnetic field lines in thecentral part of the rotor 11.

In accordance with the invention, the definition of the slots E31 and E2from the study of the spread of the magnetic field lines in the polemakes it possible to obtain a defluxing magnetic circuit with optimumdimensions. This defluxing magnetic circuit must be dimensioned so as topermit a passage of the magnetic flux surrounding the magnet PM on theappearance of a situation of a short circuit in the stator windings, orwhen in high speed a current equal as a maximum to the short-circuitcurrent is injected into the stator windings so as to oppose themagnetic flux generated by the permanent magnets.

Such a defluxing magnetic circuit prevents the appearance in the magnetPM of a demagnetising field with too high an amplitude, capable ofproducing irreversible demagnetisation of the magnet PM. The defluxingmagnetic circuit must therefore be calculated so as to obtain correctpolarisation of the magnet PM.

The equivalent magnetic circuit around a permanent magnet PM, in asituation of a short circuit in the stator windings, is represented inFIG. 4.

In a situation of a short circuit in the stator windings, the fieldlines LC (shown in FIG. 2) of the magnet PM do not pass through thestator of the machine. These field lines LC are then enclosed by meansof a high magnetic reluctance Rh and low magnetic reluctances Rb1 andRb2 of the magnetic circuit formed around the magnet PM.

The high magnetic reluctance Rh is that of the passage of the fieldlines LC through the tips B1, B2 and the recess E10. The low magneticreluctance Rb1 is that of the passage of the field lines LC through theslots E2 on either side of the bottom of the magnet PM, in the twoadjacent poles. The low magnetic reluctance Rb2 is that of the passagesof the field lines LC through the ferrous constrictions S of the twopoles, between the slots E2 and the opposing edges of the slots E1.

The constrictions formed by the portions B1, B2 and S function overallin saturation mode.

As shown in FIG. 4, the magnetic circuit comprises the source FMM inseries with the internal reluctance R_(a) and a magnetic reluctance ofthe defluxing circuit R_(f) roughly equivalent to the three reluctancesRh, Rb1 and Rb2 in parallel.

With reference also to FIG. 5, a description is now given of thefunctioning of the permanent magnet PM in a short circuit situation.

For modern permanent magnets, for example of the NdFeB type, themagnetisation curve is roughly the shape of a straight line as shown inFIG. 5. This straight line is represented by the equation:

B _(a)=μ_(a) .H _(a) +B _(r)  (1)

in which B_(a) is the magnetic induction of the magnet PM, H_(a) is themagnetic field applied to PM, B_(r) is the residual magnetic inductionof PM and μ_(a) is the PM magnetic permeability.

The tests carried out by the inventive entity have shown that in orderto avoid irreversible demagnetisation of the magnet PM, it is desirableto have a working point P satisfying the equation:

B _(a) =B _(r)/λ  (2)

with λ having a minimum value between λ_(min) and a maximum valueλ_(max), which, for magnets PM of the NdFeB type, have been determinedto be equal to:

λ_(min)=1.7 and

λ_(max)=2.5

More generally, depending on the type of magnet, λ can be between 1.3and 4.

The close value of λ=2 seems to be a good compromise in a short-circuitsituation for a magnet PM.

For simplicity, it is assumed here that the permeability μ_(a) of themagnet PM is roughly equal to the absolute permeability μ_(o)=4.π.10⁻⁷H/m.

The reluctance of a rectangular magnet such as the magnet PM is givenapproximately by the formula:

R _(a) =l _(a)/(μ_(o) .h _(a) .L _(zs))  (3)

in which, as shown in FIG. 2, l_(a) is the width of the magnet PM, h_(a)is the PM height and L_(zs) is the PM length.

The coercive field H_(c) producing final demagnetisation of the magnetPM is given in first approximation by:

H _(c) =B _(r)/μ_(a)  (4)

The magnetic flux φ_(a) generated by the magnet PM is given by theproduct of the magnetic induction and the surface, namely:

φ_(a) =B _(a) .L _(zs) .h _(a)  (5)

The magnetic flux is also given by the ratio:

φ_(a)=FMM/(R _(a) +R _(f))  (6)

The magnetomotive force of the magnet PM is given in first approximationby:

FMM=B _(r) .l _(a)/μ_(a)  (7)

From equations (5), (6) and (7), one obtains the equation:

B _(a) .L _(zs) .h _(a)=(B _(r) .l _(a)/μ_(a))/(R _(a) +R _(f))  (8)

It follows from equation (8):

R _(a) +R _(f)=(l _(a)/(μ_(a) .L _(zs) .h _(a))).(B _(r) /B _(a))  (9).

By introducing equations (2) and (3) into (9), one obtains:

R _(a) +R _(f) =R _(a)λ  (10)

This gives the ratio:

R _(f) /R _(a)=(λ−1)  (11)

In the knowledge that λ must be between λ_(min) and λ_(max), one obtainsthe inequation:

(λ_(min)−1)≦R _(f) /R _(a)≦(λ_(max)−1)  (12)

For magnets PM of the NdFeB type, with λ_(min)=1.7 and λ_(max)=2.5, oneobtains the inequation:

0.7≦R _(f) /R _(a)≦1.5  (13)

More generally, depending on the type of magnet, one obtains theinequation:

0.3≦R _(f) /R _(a)≦3  (14)

Preferably, one will choose R_(f)/R_(a)=1 which corresponds to λ=2.

In accordance with the invention, it is possible to determine valuesλ_(min) and λ_(max) for each type of magnet. Then, knowing thereluctance R_(a) of the magnets PM chosen for the machine, an overalldefluxing circuit reluctance R_(f) can be determined from the inequation(12). Once the value of the overall defluxing circuit reluctance R_(f)has been determined, it is possible to optimise the spread thereofbetween Rh, Rb1 and Rb2 so as to obtain the desired performance.

With reference now to FIGS. 6 to 10, a description is given below of theoptimisation of a portion BR forming a bridge between the magneticbodies of the rotor 11 on either side of the slot E3.

The tests carried out by the inventive entity have shown that thedimensioning of this bridge BR of the packet of rotor metal sheets isimportant in order, on one hand, to reduce the moment of inertia of therotor 11 and, on the other hand, to be able to guarantee obtaining themaximum torque wanted for the machine.

As can be seen in FIGS. 6 a and 6 b, showing two different forms ofembodiment, the bridge BR is defined dimensionally by its height H andby its length L.

Such a bridge BR is necessary for reasons of mechanical resistance ofthe rotor 11. In the forms of embodiment in FIGS. 6 a and 6 b, thebridge BR is continuous over the entire axial length of the rotor 11,such that the slot E3 does not emerge at any point of thecircumferential surface of the rotor 11. In the form of embodiment ofFIG. 6 b, the bridge BR is set back in relation to the circumferentialsurface of the rotor and a wider gap is obtained in the bridge BRbetween the stator 10 and the rotor 11. In other forms of embodiment ofthe invention not represented, the bridge BR can be made discontinuousby letting the slot E3 emerge on the exterior, at the circumferentialsurface of the rotor 11, for example, one metal sheet in two.

It will be noted that the form of embodiment of FIG. 6 b with a bridgeBR set back and the forms of embodiment indicated above with adiscontinuous bridge BR can be interesting in certain applications, inorder to reduce iron losses by a reduction of harmonics in themagnetomotive force. In the forms of embodiment described here, thevariable gap between the rotor 11 and the stator 10, visible in FIGS. 1,6 a and 6 b, also contributes to a reduction of harmonics and thereforein iron losses.

With reference to FIGS. 7 and 8, the tests carried out by the inventiveentity show that a good compromise between a maximum torque (Cmax) and aminimum inertia (I) will be obtained for a height H of the bridge BRwhich must remain less than approximately 1 times the width I_(a)(1×I_(a)) of the permanent magnets PM.

As shown in FIG. 7, the torque Cmax is achieved for a height H of1×I_(a). Moreover, as can be seen in FIG. 8, a height H greater than1×I_(a) will only result in increasing the inertia I without any gain oftorque.

On a practical level, bearing in mind the rules for cutting metal sheetsindicated earlier in the description, the height H of the bridge BR willbe between approximately 1 times the thickness of the metal sheet andapproximately 1 times the width of the magnet I_(a). Thus, for example,with metal sheets having a thickness of 0.35 mm and magnets having awidth of 5 mm, the height of the bridge BR according to the inventionwill here be between approximately 0.35 mm and approximately 5 mm.

FIGS. 9 and 10 show curves of the torque Cmax and inertia I as afunction of the length L of the bridge BR. The tests carried out by theinventive entity show that the best compromise of performance between amaximum torque and a minimum inertia will be obtained for a length L ofthe bridge BR of less than approximately 1.5 times the width I_(a)(1×I_(a)) of the permanent magnets PM. On a practical level the minimumlength L which the bridge BR can take will be of the order of 1.5 timesthe thickness of the metal sheet, namely 0.520 mm approximately for ametal sheet thickness of 0.35 mm.

The invention has been described here in the context of particular formsof embodiment. The invention will find significant applications inelectric traction engines used in cars, for electric vehicles and hybridvehicles. However, it must be clear that the invention will also findapplications in fields other than the automotive field.

1. A rotating electrical machine comprising: a stator (10) equipped withstator windings and a rotor (11), said rotor comprising a plurality ofalternate North poles and South poles which are formed from permanentmagnets (PM) arranged according to a roughly radial configuration so asto aid flux concentration, first slots (E1) arranged in the magneticbody of said rotor (11) in which are housed respectively said permanentmagnets (PM); said rotor further comprising, in each of its poles, asecond slot (E3) arranged roughly radially in the magnetic body of thepole concerned, between two consecutive of said permanent magnets (PM)delimiting said pole, said second slot (E3) realizing a reduction in themechanical inertia of said rotor (11); said rotor further comprising, ineach of its poles, a portion forming a bridge (BR) between magneticbodies of said rotor (11) on either side of said second slot (E3), andsaid permanent magnets (PM) being approximately rectangular in shapewith a height (h_(a)) in the radial direction of said rotor (11), alength (L_(zs)) in the axial direction of said rotor (11) and a width(l_(a)) having predetermined dimensional values, characterised in thatsaid part forming a bridge (BR) comprises a magnetic body height (H) notgreater than approximately one times the dimensional value of said width(l_(a)) of said permanent magnets (PM).
 2. A rotating electrical machineaccording to claim 1, characterised in that said part forming a bridge(BR) comprises a magnetic body height (H) greater than approximately onetimes the a unit metal sheet thickness of a packet of metal sheetsforming the magnetic body of said rotor (11).
 3. A rotating electricalmachine according to claim 1, characterised in that said permanentmagnets (PM) are approximately rectangular in shape with a height(h_(a)) in the radial direction of said rotor (11), a length (L_(zs)) inthe axial direction of said rotor (11) and a width (l_(a)) havingpredetermined dimensional values, and in that said part forming a bridge(BR) comprises a magnetic body length (L) less than approximately oneand a half times the dimensional value of said width (l_(a)) of saidpermanent magnets (PM).
 4. A rotating electrical machine according toclaim 1, characterised in that said part forming a bridge (BR) comprisesa magnetic body length (L) greater than approximately one and a halftimes the unit metal sheet thickness of a packet of metal sheets formingthe magnetic body of said rotor (11).
 5. A rotating electrical machineaccording to any one of claim 1, characterised in that said part forminga bridge (BR) is set back in relation to the circumferential surface ofsaid rotor (11).
 6. A rotating electrical machine according to any oneof claim 1, characterised in that said second slot (E3) comprises a toppart (E30) which participates in a reduction in the mechanical inertiaof said rotor (11) and a bottom part (E31) which participates incontrolling a defluxing magnetic flux through the central part of therotor (11).
 7. A rotating electrical machine according to claim 6,characterised in that said top part (E30) has a approximatelytrapezoidal shape with a smaller side corresponding roughly to said partforming a bridge (BR).
 8. A rotating electrical machine according to anyone of claim 1, characterised in that said second slot (E3) isapproximately off-centre, between said two consecutive permanent magnets(PM) delimiting said pole.
 9. A rotating electrical machine according toclaim 1, characterised in that said rotor (11) comprises, on either sideof a said second slot (E3), at least one third slot (E4) reducing theinertia of said rotor (11) and oriented approximately within thealignment of magnetic field lines.
 10. A method for dimensioning a rotorof a rotating electrical machine comprising a stator (10) equipped withstator windings and a rotor (11), said rotor comprising a plurality ofalternate North poles and South poles which are formed from permanentmagnets (PM) arranged according to a roughly radial configuration knownas with flux concentration, first slots (E1) arranged in the magneticbody of said rotor (11) and in which are housed respectively saidpermanent magnets (PM); said rotor comprising, in each of its poles, asecond slot (E3) arranged roughly radially in the magnetic body of thepole concerned, between two said consecutive permanent magnets (PM)delimiting said pole, said second slot (E3) participating in a reductionin the mechanical inertia of said rotor (11); and said rotor comprising,in each of its poles, a portion forming a bridge (BR) between themagnetic bodies of said rotor (11) on either side of said second slot(E3), and selecting said part forming a bridge (BR) such that saidbridge is defined dimensionally (H, L) such that said bridge (BR)comprises a magnetic body height (H) not greater than approximately onetimes the dimensional value of a width (l_(a)) of said permanent magnets(PM) and a magnetic body length (L) less than approximately one and ahalf times the dimensional value of said width of said permanentmagnets, so as to obtain an optimum compromise between a maximummechanical torque (Cmax) provided by the machine and a minimummechanical inertia of said rotor (11).